A plan to slash more than $500 billion from future Medicare spending -- one of the biggest sources of funding for President Obama's proposed overhaul of the nation's health-care system -- would sharply reduce benefits for some senior citizens and could jeopardize access to care for millions of others, according to a government evaluation released Saturday.

The report, requested by House Republicans, found that Medicare cuts contained in the health package approved by the House on Nov. 7 are likely to prove so costly to hospitals and nursing homes that they could stop taking Medicare altogether.

Congress could intervene to avoid such an outcome, but "so doing would likely result in significantly smaller actual savings" than is currently projected, according to the analysis by the chief actuary for the agency that administers Medicare and Medicaid. That would wipe out a big chunk of the financing for the health-care reform package, which is projected to cost $1.05 trillion over the next decade.

More generally, the report questions whether the country's network of doctors and hospitals would be able to cope with the effects of a reform package expected to add more than 30 million people to the ranks of the insured, many of them through Medicaid, the public health program for the poor.

In the face of greatly increased demand for services, providers are likely to charge higher fees or take patients with better-paying private insurance over Medicaid recipients, "exacerbating existing access problems" in that program, according to the report from Richard S. Foster of the Centers for Medicare and Medicaid Services.

Though the report does not attempt to quantify that impact, Foster writes: "It is reasonable to expect that a significant portion of the increased demand for Medicaid would not be realized."

The report offers the clearest and most authoritative assessment to date of the effect that Democratic health reform proposals would have on Medicare and Medicaid, the nation's largest public health programs. It analyzes the House bill, but the Senate is also expected to rely on hundreds of billions of dollars in Medicare cuts to finance the package that Majority Leader Harry M. Reid (D-Nev.) hopes to take to the floor this week. Like the House, the Senate is expected to propose adding millions of people to Medicaid.

The Centers for Medicare and Medicaid Services administers the two health-care programs. Foster's office acts as an independent technical adviser, serving both the administration and Congress. In that sense, it is similar to the nonpartisan Congressional Budget Office, which also has questioned the sustainability of proposed Medicare cuts.

In its most recent analysis of the House bill, the CBO noted that Medicare spending per beneficiary would have to grow at roughly half the rate it has over the past two decades to meet the measure's savings targets, a dramatic reduction that many budget and health policy experts consider unrealistic.

"This report confirms what virtually every independent expert has been saying: [House] Speaker [Nancy] Pelosi's health-care bill will increase costs, not decrease them," said Rep. Dave Camp (Mich.), the senior Republican on the House Ways and Means Committee. "This is a stark warning to every Republican, Democrat and independent worried about the financial future of this nation."

Democrats focused Saturday on the positive aspects of the report, noting that Foster concludes that overall national spending on health care would increase by a little more than 1 percent over the next decade, even though millions of additional people would gain insurance. Out-of-pocket spending would decline more than $200 billion by 2019, with the government picking up much of that. The Medicare savings, if they materialized, would extend the life of that program by five years, meaning it would not begin to require cash infusions until 2022.

"The president has made it clear that health insurance reform will protect and strengthen Medicare," said White House spokeswoman Linda Douglass. "And he has also made clear that no guaranteed Medicare benefits will be cut."

Republicans argued that the report forecasts an increase in total health-care spending of more than $289 billion.